This invention relates to a piston-cylinder assembly of plastic material, of glass fibre reinforced plastic, for a master assembly for hydraulic systems in automotive vehicles, and to a process and an apparatus for its manufacture.
In the published German patent application No. 3149628.8, a ball-and-socket joint connection is described between a push rod furnished with a ball head and a pressure element, between the push rod and a piston of a hydraulic brake power booster or master cylinder. The push rod is swingably inserted in a base bore of the piston and a retaining element which simultaneously serves as a reinforcing element and partly surrounds the ball head being arranged between the base bore and the ball head. It is proposed in the patent application that the retaining element is provided with a plurality of elastic projections some of which are formed with offsets which back-grip the ball, whereas the others abut with an initial stress against the wall of the base bore.
In International Patent Application No. WO 87/03344 (PCT application) a cylinder is disclosed which is applied in a hydraulic system. The cylinder is composed of two parts. Each part is made of plastic material. One part comprises an external shell which presents an opening. An end wall is provided at the opposite end. The other part comprises an internal bushing. The bushing is precision-fitted within the shell. It extends from the forementioned open end to the fore-mentioned end wall. The end of the forementioned bushing which is disposed distant from the open end is located at a distance from the end wall. A cylindrical bore is provided in the bushing. A piston is slidingly arranged within the bore. The piston can move in the direction of the end wall and away from the end wall.
Sealing means are provided which abut against the circumference of the piston. The sealing means are retained between the internal end of the bushing and the opposite surface of the shell. A pressure chamber is arranged between the end wall and the piston.
Furthermore, a hydraulic master cylinder is described in the British patent application No. GB 2071797A which comprises a cylinder and a supply tank manufactured separately. The supply tank is fastened on the cylinder. The supply tank and the cylinder are interconnected in such a manner that hydraulic fluid may flow between the supply tank and the cylinder during operation. The supply tank has projecting arms which are in abutment against the outside cylinder wall, against opposite sides of the cylinder. These arms serve to keep the supply tank and the cylinder together, so that they can be put into operation jointly. The supply tank is made of plastic material.
Known piston-cylinder assemblies have a complicated set-up which is unsuitable for mass production. In many cases, metal supporting or reinforcing elements are required, since the plastic bodies alone are not capable of absorbing the mechanical forces. In addition, the known assemblies require expensive mechanical machining.
Accordingly, the invention has the following objects: In piston-cylinder assemblies, particularly in clutch master assemblies, the sealing cup which serves to seal the piston within the cylinder frequently is passed over uncontrollably. In many cases, the reason is that the cup has an axial play. It is one of the objects of the invention to achieve a positioning of the cup without such axial play. It has been proposed to provide springs and sheet metal elements to exert an axial pressure on the cup and, thus, to contribute to the positioning of the cup. The combination of plastic material elements and sheet metal elements is unfavorable because this leads to malfunctioning and reduces the service life of the piston-cylinder assembly. In addition, such a combination is expensive, in particular for mass production, and does not allow automation with regard to the manufacture and to mounting. Therefore, it is another object of the present invention to render such sheet metal elements and springs superfluous. Furthermore, the other metal components, such as retaining metal sheets, cup elements, etc. which are to reinforce and support, which are utilized in the known assemblies are avoided.
It is, moreover, an object of the present invention to use phenolic resin plastic as a material to construct a piston-cylinder assembly.
In addition, fewer components than in known devices are utilized for the set-up of the piston-cylinder assembly. The costs of manufacture thereof are reduced. Further, the invention should provide the possibility of connecting the cylinder of the assembly, if it is mounted in the passenger compartment, to the hydraulic system from the engine compartment.
Glass fibre reinforced plastic is machined only with difficulty. It is not weldable. These inconveniences which are due to the material are avoided by the present invention. In particular, the invention safely absorbs the forces caused by the high pressure in the cylinder through a plastic material thread. Manufacture and mounting of the entire piston-cylinder assembly are easily accomplished. In particular, favorable conditions are created for a mechanical and automated mass production.
The cutting or machining operations which were necessary in known piston-cylinder assemblies are reduced considerably by the present invention. The breather bore of known master cylinder assemblies are avoided. In the known devices, the breather bore represents a problematic detail which leads to heavy wear of the cups in the piston-cylinder assembly. The known master cylinders comprise numerous components which require in many cases machining in a highly expensive manner. The present invention has the further object to considerably reduce the number of components required.
The manufacture of the master cylinder should involve lower costs than known devices. The inventive master cylinder provides a design which offers ease of mounting and is, in particular, suitable for a low-cost mass production. The present invention provides conditions for a mechanical final mounting of the assembly in its entirety.
Duroplast material is applied at an increasing extent as a material for the manufacture of plastic cylinders. In many cases, duroplast is reinforced with glass fibres. Mechanical machining of duroplast, in particular if it is reinforced with glass fibres, is expensive and difficult. The service lives of the tools are in part unreasonably short. Consequently, it is another object of the present invention to provide processes and apparatus for the realization of the processes which either avoid any mechanical machining or reduce machining to a significant extent.
In accordance with the present invention, these prerequisites are fulfilled in that the plastic components of the master assembly do not require any supporting or reinforcing metal elements as is the case of known plastic cylinders. Among others, it is an object of the invention to utilize the duroplast piston material by itself, that is, without metal auxiliary means for securing the ball-shaped head of the piston rod in the piston. This means that no clips and springs are necessary as is the case in known devices to connect the piston rod to the piston. In this regard, such metal elements cannot be used in combination with duroplast as duroplast is to brittle.
Strict observance of the dimensions of the inside contours of the housings of cylinders which are made of plastic material is problematic in the known devices. This applies, in particular, if and when the housings are manufactured by plastic injection molding processes in mass production. It is an object of the present invention to achieve a higher dimensional accuracy of the inside contours of the housings. It is a further object of the present invention to improve and to reduce the cost of mass production of plastic cylinders, in particular of plastic master cylinders for the automotive vehicle technology, while simultaneously maintaining an elevated dimensional accuracy of the inside contours. Moreover, conditions are provided by the present invention which render it possible to manufacture the entire cylinder housing in mass production in its final shape with dimensionally accurate inside contours substantially by plastic injection molding alone. Any cutting or machining is, for example, avoided.
It is another object of the present invention that bosses, burrs, in particular mold division burrs which come about when two-part tools are used do not introduce problems in the event of subsequent assembly of the cylinder or impair the functions of the cylinder. As an alternative, the present invention provides that in the course of the manufacturing process such bosses, burrs, etc. are eliminated, and, particularly, sheared off.